1. Technical Field of the Invention
The invention relates to a method for operating an interface device, an interface device, and an exchange with such an interface device.
2. Discussion of Related Art
A large segment of the today's worldwide telecommunication network is operated in synchronous time division multiplex mode. Long distance calls which represent the major portion of the traffic in this network, are transmitted in communication channels at 64 kbit/s. These channels are combined in different hierarchical orders into channel bundles with a plurality of channels. In Europe and elsewhere, the next hierarchical order is located at two Mbit/s (exactly at 2.048 Mbit/s). This so-called primary multiplex mode (E1) contains 30 voice channels, one synchronization channel, and one signaling channel, each with 64 kbit/s. In this synchronous time division multiplex mode, there exists a defined temporal correspondence between time slots and transmission channels. A similar hierarchical level called DS1 (Digital Signal-Level 1) is used in the USA and elsewhere at 1.544 Mbit/sec with 24 voice channels.
Since some time, these telecommunication networks are also utilized for other purposes besides voice transmission. For many of today's applications (an important application of this type is telefax), such a voice channel with 64 kbit/s is utilized as a data channel. It is unimportant for the present invention if these data are supplied to the network directly in digital form or by way of an analog connection via modem.
The transmission capacity of 64 kbit/s which is adapted to voice transmission, forms inevitably also the basis for data transmission. Data transmission can, to a certain extent, be adapted to available transmission channels. There exist, however, certain limitations. For this reason, there have been established, on one hand, specialized data networks which are somehow connected to the remaining communication network and consequently are subnetworks thereof. On the other hand, several individual channels can be occupied at the same time and used as a transmission channel with higher capacity.
The transmission channels which are used here primarily, correspond to the higher multiplex levels, for example, two Mbit/s. But also other bundles, such as n.times.64 kbit/s and n.times.2 Mbit/s or n.times.1.5 Mbit/s, can be advantageously applied.
A completely different approach for transmitting data is provided by the statistical time division multiplex technique. This technique was originally developed as a packet switching technique specifically for transmitting data. When data are to be transmitted, these data are always combined in a packet and sent to the recipient preceded by an address segment. This can be accomplished in asynchronous time-division multiplex mode but also in other multiplex modes capable of subdividing the available capacity. Today's ATM technology (ATM=asynchronous transfer mode) was developed on this foundation.
Each connection is preceded by a connection setup phase wherein, besides other tasks, a route is established from the calling party to the called party. This facilitates addressing and enables relative and thereby shorter addresses. A prerequisite, however, is the availability of the necessary transmission capacity. For this purpose, a certain transmission capacity must be associated with each connection by agreement. In modern ATM technology, this is accomplished, for example, by specifying an average data rate, a maximum data rate, and a traffic type such as "constant data rate" or "variable data riate".
Initially, it is unimportant how this assignment is made. One possibility are one-time agreements which are then valid for all connections of the subscriber; the transmission capacity between the subscriber device and the exchange is automatically allocated, preferably during each connection setup; however, adaptive changes can be made during the connection and are not excluded. Today, agreements are preferred wherein the deviations from a time averaged value are as small as possible. Agreements with statistical variations are generally not excluded. The available reserved transmission capacity can be utilized economically through buffering or adaptive changes to the agreement. All agreements, including any changes, however, have to be routed onward by way of signaling to all affected locations along the transmission path.
The worldwide telecommunication network discussed above will, however, be always subdivided into subsets employing different technologies and different standards. These subnetworks cooperate via interface devices. The signaling between the two subnetworks with their different signaling protocols is also converted.
The version of the ITU-T recommendation I.580, COM 13-R 30-E, which was ratified in December 1994, was published by the ITU (International Telecommunications Union) on Feb. 9, 1995. The recommendation relates to "General arrangements for interworking between B-ISDN and 64 kbit/s based ISDN".
B-ISDN herein refers to the statistical multiplex technique referenced above as ATM. ISDN refers to the synchronous time division multiplex technique. The recommendation cited above describes an "Interworking Function" between ATM and ISDN with n.times.64 kbit/s. The case n&gt;1 is mentioned therein, but is not described in greater detail.
With this technique, it is possible to connect each transmission channel on the ISDN side with a virtual channel on the ATM side. It is also known, for example, that a complete primary multiplex mode including 30 information channels, one synchronization channel, and one signaling channel can all together be connected with one virtual channel on the ATM side.